I agree with those that state there is no set way. To say one way is "proper" is misleading.
There has been many pages written about this issue. Most of the people in-the-know would agree that there is no set way.
Concerns include a metal faceplate comeing loose, and falling down into the hot plug blade. I would think that when the faceplate falls down, it can only fall one way or the other, so it will contact either the grounded(neutral) conductor, or the grounding conductor. Either will cause the breaker to trip. With the ground up, the faceplate will land on the grounding blade, and, if falling into the hot blade, will trip the breaker. If, however, it rotates the other way, into the neutral blade, then no breaker action will occur. Is this a good enough reason to specify one way over another? I don't think so.
So I like my outlets with the ground down, just because it looks like a face-----As good a reason as most others!!